Image plus 2

A Witec confocal Raman microscopy α300R (Witec, Ulm, Germany) was used to collect the Raman spectra. The confocal Raman microscopy is equipped by a frequency doubled Nd: YAG laser (Newport, Evry, France) with the wavelength of 532 nm and a ×20 NIKON air objective with numerical aperture of 0.46 (Nikon, Tokyo, Japan). The output laser power is 50 mW. The spatial resolution is 300 nm and depth resolution is roughly 1 μm. The microscope is equipped with a piezodriven scan stage with a positioning accuracy of 2 to 3 nm horizontally and 10 nm vertically. The acquisition time of a single spectrum was set to 0.5 s. 150 × 150 points per image were recorded, leading to a total of 22,500 spectra for one image. Data acquisition is performed using Image Plus 2.08 software from Witec. Using an edge filter, Raman backscattered radiations are separated from the Rayleigh scattered light. The Raman photons are transferred to the electron multiplying charge-coupled device camera EMCCD (DU 970N-BV353, Andor, Hartford). The EMCCD chip size is 1600 × 200 pixels, the camera controller is a 16 bit A/D converter operating at 2.5 MHz, and the camera is cooled by a Pelletier system. The UHTS 300 spectroscopy system with a 70% throughput transmission and a grating of 600 lines∕mm (operating at -60°C) provide a spectral resolution of 3 to 5 cm -1 .